SSD for PCI-Express: OCZ IBIS vs. OCZ RevoDrive. Page 3

Today we are going to talk about SSDs with PCI –Express interface. This type of devices is free from a common bottleneck – insufficiently fast SATA interface, so this is where we should be looking for a performance leader.

SSDs are tested with generic OS drivers. We format them as one NTFS partition with the default cluster size (for FC-Test we create two 32GB partitions). We have transitioned to new testing methodology, by the way.

Performance in Intel IOMeter

Sequential Read & Write Patterns

IOMeter is sending a stream of read and write requests with a request queue depth of 4. The size of the requested data block is changed each minute, so that we could see the dependence of an SSD’s sequential read/write speed on the size of the data block. This test is indicative of the maximum speed the tested SSD can achieve.

The numeric data can be viewed in tables by clicking the links below. We will be discussing graphs and diagrams.

Although somewhat lower than specified, the sequential speeds are indeed very, very high. The RevoDrive x2 and the IBIS series, excepting the 160GB model, hit the 600MBps mark. Yes, each of these small quad-controller SSDs would outperform a RAID0 array built out of four modern 15,000RPM hard disks. This is very, very fast. The RevoDrives with only two flash controllers are slower, delivering a top read speed of 400 MBps. And our custom-made single-controller version is no faster than 200 MBps. So, the performance of these SSDs scales up depending to the number of memory access channels since they are not limited by the interface bandwidth or by the RAID controller. It must be noted that the difference in speed can only be felt on 32KB or larger data blocks. The top speeds are achieved rather late into the test. In other words, your applications must be able to work with large data blocks for these SSDs to show their best.

One remark is necessary about what you cannot see in the diagram. This very high performance of the disk subsystem calls for a high-performance CPU. Our testbed is equipped with a rather old but fast Pentium 4 620 processor, and its load varied from 27% (large data blocks) to 84% (small data blocks) throughout this test. Yes, even though the CPU is not heavily involved into disk operations, it takes a lot of CPU resources when there are over 30 thousand data blocks being read from the disk each second!

We’ve got very interesting results in the sequential write test. The top-capacity IBIS takes first place as it has four flash controllers with a full set of chips on each. Second place is shared by the IBIS 160 GB and the RevoDrive 240 GB. The former has four SF1200 controllers but each of these controllers only uses six out of eight memory access channels. The RevoDrive 240 GB has only two SF1200 controllers but these work with full memory sets, i.e. use all of their memory access channels. As a result, these differently designed SSDs have almost identical graphs.

With half the memory chips, the 100GB IBIS draws its graphs at about half the level of its full-capacity cousin, indicating that a direct correlation exists between the number of memory access channels and write speed. Interestingly, the RevoDrive x2 is somewhat slower than the similarly designed IBIS 100 GB and even inferior to the RevoDrive 120 GB which has only two flash controllers but a full set of memory chips. The junior models of the RevoDrive series are rather a depressing sight, yet our custom-made sample with only one memory access channel is even worse.